This invention relates to a method for the recovery of actinide, strontium and technetium values together from an aqueous acid solution. More specifically, this invention relates to a process for the separation of uranium, the transuranic (TRU) elements neptunium, plutonium and americium, together with strontium and technetium from aqueous nitric acid waste solutions containing these and other values and for the separate recovery of TRU and strontium elements from the other recovered values.
The disposition of the radioactive waste which results from the processing of nuclear material for the production of nuclear weapons is a major problem. Many of these wastes are high level (HLW) and have been stored for long periods of time in single walled underground storage tanks which, because of the extended storage period, present a potential environmental hazard. The waste consists of a high level radioactive sludge containing transuranic (TRU) elements and .sup.90 Sr and a radioactive alkaline liquid containing primarily .sup.137 Cs. Ideally, the waste stored in these tanks should be removed and processed for long-term storage in a deep geographic repository. Because of the expense associated with long-term storage, it is desirable to minimize the volume of radioactive waste by first removing the long-lived actinides, particular in neptunium, plutonium and americium. Another element which must be removed is the major heat-producing isotope .sup.90 Sr which, if left in the waste as it is solidified for storage, will produce as significant amount of heat which must be removed from the stored waste solid. The remaining waste is not sufficiently radioactive to require placement in a deep geologic repository. Only the actinide fraction will require such placement.
In present nuclear waste processing technology, the TRU values are recovered from nitric acid waste solutions by the methods described in U.S. Pat. Nos. 4,548,790 and 4,574,072, both assigned to the U.S. Department of Energy and incorporated herein by reference. U.S. Pat. No. 4,548,790 describes a method for the recovery of lanthanide and actinide values from aqueous nitric acid solutions utilizing any of various neutral bifunctional extractants, the alkyl(phenyl)-N,N-dialkylcarbamoyl-methylphosphine oxides, hereinafter referred to as CMPO's, in an appropriate diluent such as diethylbenzene. U.S. Pat. No. 4,574,072, describes an improvement in the use of CMPO for extracting lanthanide and actinide values from acid solutions by the substitution of normal paraffin hydrocarbons containing a phase modifier such as tri-n-butyl phosphate (TBP) as the diluent.
Strontium and technetium values are recovered from nitric acid waste solutions by the process described in U.S. patent application Ser. No. 506,125, filed Apr. 9, 1990 in the name of the U.S. Department of Energy and incorporated herein by reference. The process utilizes an extractant solution consisting of a macrocyclic polyether or "crown ether" such as dicyclohexano-18-crown-6 (DCH18C6) in a diluent which capable of dissolving a substantial quantity of water and in which the crown ether is soluble such as 1-octanol.
Although these processes are separately effective, they do require two completely separate processes, increasing the cost and the complexity of the recovery operation.
There is a clear need therefore, for a single recovery process which could extract the TRU values (neptunium, americium and plutonium) and the strontium values in a single operation rather than in two separate ones as is presently necessary. Such a process would require less space and equipment, would be easier to control than two separate extraction processes and would require the handling of only one process stream. However, the prior art extractants, CMPO and crown ethers are not compatible together in paraffinic hydrocarbon diluents, forming in addition to the normal aqueous and organic phases, a second organic phase. In addition to the usual problems with phase separation encountered when a third phase is present, there is a strong potential for achieving criticality since any plutonium present in the waste stream tends to concentrate in the second organic phase.
It has been found that by the careful selection of diluent, it is possible to combine the CMPO and crown ether, to prepare a new extractant solution which will extract the TRU elements (i.e. neptunium, plutonium and americium) together with strontium in a single process stream.